Question
In some countries, liquid nitrogen is used on dairy trucks instead of mechanical refrigerators. A 3.00-hour delivery trip requires 200 L of liquid nitrogen, which has a density of $808\textrm{ kg/m}^3$. (a) Calculate the heat transfer necessary to evaporate this amount of liquid nitrogen and raise its temperature to $3.00^\circ\textrm{C}$ . (Use $c_p$ and assume it is constant over the temperature range.) This value is the amount of cooling the liquid nitrogen supplies. (b) What is this heat transfer rate in kilowatt-hours? (c) Compare the amount of cooling obtained from melting an identical mass of $0^\circ\textrm{C}$ ice with that from evaporating the liquid nitrogen.
1. $6.59\times 10^{7}\textrm{ J}$
2. $18.3\textrm{ kW}\cdot\textrm{h}$
3. Liquid nitrogen has a cooling effect 22% greater than that of an equal mass of melting ice.

# OpenStax College Physics for AP® Courses, Chapter 14, Problem 28 (Problems & Exercises)

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